1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor and an electrophotographic image forming apparatus, and more particularly, to an electrophotographic photoreceptor having high photosensitivity and good stability over repeated charging. The invention is also directed to an image forming apparatus employing the electrophotographic photoreceptor.
2. Description of the Related Art
Phthalocyanine compounds show good photoconductivity to light in the range of visible light to near infrared and thus are widely used as a photoelectric material for a charge generating material of electrophotographic photoreceptors or organic solar batteries. Titanyl phthalocyanine compounds having a tetravalent titanium atom bonded with a hydrogen molecule as a main metal are used mostly for their good photosensitivity and stability.
Like many other phthalocyanine compounds, titanyl phthalocyanine compounds have various crystal forms at room temperature.
For example, U.S. Pat. No. 4,664,997 discloses a titanyl phthalocyanine crystal having a major absorption peak at a wavelength of around 760 nm. This crystal type is generally known as a β-type and has the most stability and the lowest photosensitivity among currently used titanyl phthalocyanine crystals.
U.S. Pat. No. 4,728,592 discloses an α-type titanyl phthalocyanine crystal having a major absorption peak at a wavelength of around 830 nm. The photosensitivity of the α-type titanyl phthalocyanine crystal is 1.5 times greater than that of the β-type titanyl phthalocyanine crystal, which is effective for obtaining a high efficiency electrophotographic photoreceptor.
U.S. Pat. No. 4,898,799 discloses a phthalocyanine crystal type having a major absorption peak at a Bragg angle (2θ) of 27.3° in an X-ray diffraction spectrum. This crystal type is generally called a Y-type or γ-type, and has high quantum efficiency of 90% or more at the intensity of an ordinary electrical field and is practically utilized for a super high photosensitivity photoreceptor. The Y-type crystal shows a plurality of major absorption peaks in a long wavelength range, and has generally an absorption peak at a wavelength of around 800 nm and around 850 nm, and their intensity ratio can vary according to manufacturing conditions. This crystal type is quasi-stable, and is likely to be stabilized by being exposed to heat or a mechanical stress, or contact with a solvent, thereby reducing the photosensitivity. Also, this crystal type is known to include water molecules in the crystal structure and thus its property varies according to the humidity of the environment.
Also, U.S. Pat. No. 5,252,417 discloses a titanyl phthalocyanine crystal obtained from an amorphous titanyl phthalocyanine treated with monochlorobenzene and water. The titanyl phthalocyanine crystal has a major absorption peak at a Bragg angle (2θ) of about 27.3° in an X-ray diffraction spectrum like a Y-type titanyl phthalocyanine crystal, but shows a different visible-infrared absorption spectrum, and thus has a major absorption peak at a wavelength of around 790 nm and a minor absorption peak at around 710 nm.
However, when such highly sensitive titanyl phthalocyanine charge generating materials are used, the sensitivity of the photoreceptor may increase, but this increase may not be sufficient and a charge transporting material to be used in combination with the titanyl phthalocyanine charge generating material should be selected appropriately. When the combination of the charge generating material and the charge transporting material is not appropriate, the highly photosensitive titanyl phthalocyanine charge generating material cannot properly fulfill its function, and thus even when a highly photosensitive titanyl phthalocyanine charge generating material is used, the photosensitivity of the electrophotographic photoreceptor manufactured using the charge generating material is not sufficient and/or the stability over repeated charging of the photoreceptor may rapidly decrease.